This particular aspect of the spline series' appearance is not modifiable. What you see is a direct consequence of the algorithm used to render a spline which is nearly ubiquitous for all spline implementations. For more information, you can check out this page
that explains how splines are calculated and rendered at runtime. In addition, you can check out this demo
that shows the changes in a spline in real time depending on the distance between points.
In short, the reason why the spline goes backwards is because there is a set of three points, where the middle point is very close to one of the rest and its other neighbor is too far away. Of course, you could trick the chart's spline series into appearing the way you require by rearranging the data points, i.e. changing the source data. But, needless to say, I don't think that is a feasible solution.
Another option would be to use a different series, such as a line, as you mentioned initially.
the Telerik team